This invention relates generally to the mechanical bonding together of two or more material webs, and more particularly to apparatus and methods for increasing the bond strength between mechanically bonded web materials.
Mechanical bonding is a conventional bonding technique used to quickly bond together two or more webs or layers of material. One example of a mechanical bonding technique is commonly referred to as ultrasonic bonding in which the material webs comprise at in part a thermoplastic material that when exposed to a high frequency vibration, such as ultrasonic vibration, the thermoplastic materials of each web are heated, slightly melted and flow into each other to form a mechanical and/or chemical bond to hold the two webs together. As an example, ultrasonic bonding is common in the manufacturing of absorbent articles, which find widespread use as personal care products such as diapers, children's toilet training pants, adult incontinence garments, sanitary napkins and medical garments and the like, as well as surgical bandages and sponges.
One commonly employed ultrasonic bonding apparatus includes a rotatably driven ultrasonic horn and a corresponding rotatably driven anvil in closely spaced relationship with the horn to define a nip therebetween. In some ultrasonic bonding apparatus the anvil and horn contact each other at the nip while in other ultrasonic bonding apparatus typically referred to as a fixed gap, or non-contact ultrasonic bonding apparatus the anvil and horn are held in spaced relationship with each other at the nip. Two or more material webs that are to be bonded together are fed to the nip in opposed (e.g., overlaid) relationship. As the webs pass through the nip, the ultrasonic horn is excited at an ultrasonic frequency, typically in the range of about 15 kHz to about 40 kHz to ultrasonically bond the material webs together.
Because the ultrasonic vibration quickly heats and sufficiently melts the material webs to bond the webs together, high speed manufacturing is possible. For example, the material webs can be fed to the nip between the horn and anvil at speeds of at least 100 feet per minute and it is commonly faster, such as 800 feet per minute, 1,000 feet per minute or more. The linear speeds of the horn and anvil, i.e., the tangential speeds of the horn and anvil at the nip, typically match the rate at which the material webs are fed to the nip.
With recent advancements toward the use of more biodegradable materials in absorbent products, materials such as polylactic acid fibers may now be used in forming material webs for disposable absorbent articles. Polylactic acid fibers are generally known to have a relatively high stiffness and resiliency, a narrow softening range (also referred to herein as a melting band), and a high glass transition temperature as compared to more commonly used thermoplastic materials. One drawback that has come about as a result of shifting toward such materials is that these materials are less susceptible to being ultrasonically bonded together using existing bonding approaches.